Due to the increased demand for higher recording density of a magnetic recording medium, thin metal film type magnetic recording mediums have drawn much attention a compared to conventional coated type magnetic recording mediums.
However, unlike the conventional coated type recording mediums, it has been difficult to incorporate a lubricating agent and an abrasive agent into a magnetic layer of thin metal film type magnetic recording mediums. Therefore, running durability has been found to be extremely poor. For example, a perpendicular magnetic recording disk having a CO--Cr alloy film can not continue running after 5000 passes or more in many cases, because the alloy layer sticks to the magnetic head.
To solve this problem, it has been proposed to coat a liquid lubricating agent on a surface of a magnetic layer, or alternatively to provide a solid protective layer on a surface of a magnetic layer. In the latter instance, in order to remove stains present on the surface of a thin metal film and to improve the adhesiveness of a protective layer to a thin metal film, it has been proposed to first wash the surface of a thin metal film with an alkaline solution, as disclosed in JP-A-62-42331. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
It is known that providing a protective layer mainly composed of carbon, which is solid lubricating agent, on a surface of a magnetic layer is particularly effective in improving running durability, as disclosed in JP-B-54-33521. (The term "JP-B" as used herein means an "examined Japanese patent publication".)
However, a protective layer having a conventional structure and comprised of conventional materials has proved inadequate in terms of imparting sufficiently high durability, wear resistance, weather resistance and corrosive resistance to the magnetic recording medium. Also, a head sticks to the surface of the medium, which is the so-called "adsorption" phenomenon.
In this connection, it has been proposed to provide a top coat layer containing a mono- or polyether having a perfluoroalkyl group, a carboxylic acid derivative thereof or a phosphoric acid derivative thereof as a lubricating agent on a surface of such a protective layer, as disclosed in JP-A-62-150525.
Still, running durability has been found to be insufficient even if a top coat layer is provided on a protective layer mainly composed of carbon because the top coat layer provides insufficient lubrication and attachments present on a head increase the friction coefficient of the protective layer.
Moreover, although running durability is slightly improved by alkaline washing the surface of a thin metal film, such treatment is insufficient to provide a high density recording medium with high durability. Further, as the number of running passes increases, the motor torque undesirably increases.